We now explain the idea behind the proofs of the main theorems.
The critical step is Theorem REF  1; the other steps follow from this using rather standard argument.
We use the lower Dirichlet form \( (\underline{\mathcal {E} }, \underline{\mathcal {D}})\)  introduced in Lemma REF .
This Dirichlet form satisfies the relation [1]}, [2]}
\(&(\underline{\mathcal {E} }, \underline{\mathcal {D}}) \le (\mathcal {E} , \mathcal {D})\) 
